Halogen lamps that operate at a rated voltage or 12 V are in widespread use. The power supply for such halogen lamps is often produced by stepping down a line supply voltage. Since a conventional step-down transformer is bulky, a device (hereinafter referred to as an electronic transformer) is employed that converts the AC line supply voltage to a high-frequency voltage by using an inverter circuit and then converts the voltage to a lower voltage by using a high-frequency transformer. The inverter circuit to be used in the electronic transformer is one of two types, a self-excited oscillation type or an externally excited oscillation type (refer, for example, to FIG. 6 in patent document 1).
FIG. 12 is a diagram showing a lighting device equipped with a self-excited inverter as presented in the description of the prior art in patent document 1.
The lighting device comprises an AC power supply 1, a full-wave rectifier 2, a halogen lamp 14, a half-bridge inverter circuit, and other circuit elements, and the entire circuitry, excluding the AC power supply 1 and the halogen lamp 14, corresponds to the electronic transformer. A circuit comprising a resistor 5, a capacitor 6, and a triggering device 7 outputs an oscillation initiating trigger when a predetermined voltage is reached. Self-excited oscillation occurs within a circuit comprising capacitors 3 and 4, transistors 8 and 9, and a current feedback transformer 13. A high-frequency step-down, transformer 12 produces an AC drive voltage for the halogen lamp 14. Diodes 10 and 11 are protection diodes for providing protection against back electromotive force. The lighting device depicted in FIG. 12 shows the operating principle of the half-bridge inverter circuit.
The invention disclosed in patent document 1 is intended to suppress the rush current that occurs during startup, and for that purpose, a soft start circuit A and a voltage feedback circuit B are provided as shown in FIG. 1 included in patent document 1.
Recently, LED illumination devices have begun to be used widely, replacing not only incandescent lamps but also halogen lamps. Conventional halogen lamps have power ratings of 20 to 40 W but, in the case of LED illumination devices, brightness comparable to halogen lamps can be obtained at 4 W which is about one-fifths of the above power rating. However, if the self-excited electronic transformer is inserted between the AC line power supply and the halogen lamp bayonet, base (MR16 or MR11 in the case of 12-V halogen lamps), as described above, the LED illumination device may not light (or may turn on and off erratically).
Electronic transformers widely used for halogen lamps range from small ones for driving single halogen lamps to large ones rated for 100 to 300 W capable of driving several halogen lamps. For these electronic transformers, the minimum operating power necessary for startup is specified in order to produce a stable output of 12 VAC, and generally a circuit load of 10 to 30 W is required. When an LED illumination device is connected to such an electronic transformer (for example, in FIG. 12, the LED illumination device is connected to the secondary side n2 of the high-frequency step-down transformer 12), the output of the electronic transformer may become unstable due to an insufficient load, causing the LED illumination device to malfunction by failing to light or turning on and off erratically.
In view of the above, there is proposed a circuit that ensures stable operation when the LED illumination device is connected, regardless of whether the inverter circuit used for the 12-V halogen lamp power supply is of the self-excited oscillation type or the externally excited oscillation type (patent document 2).
FIG. 13 is a diagram redrawn from FIG. 2 from patent document 2.
In FIG. 13, the output of a self-excited electronic transformer (inverter-type voltage conversion circuit) (in the case of FIG. 12, the secondary side of the high-frequency step-down transformer 12) is connected to a terminal U-IN of a power feed unit 30. A lighting circuit 40 first supplies current to a startup assisting circuit 42, inducing self-excited oscillation in the electronic transformer. Next, a constant-current load circuit 43 draws a constant current from the electronic transformer to stabilize the oscillation. After that, the lighting circuit 40 causes the LED 22 to light, while at the same time, stopping the operation of the constant-current load circuit 43.
Patent document 1: Japanese unexamined Patent Publication No. H02-66890 (FIGS. 1 and 6)
Patent document 2: Japanese Unexamined Parent Publication No. 2010-135139 (FIG. 2)